1. Field of the Invention
The present invention relates to a metal separator for solid polymer electrolyte fuel cells, and relates to a production method for the separator.
2. Description of the Related Art
In a solid polymer fuel cell, a laminated body, in which a separator is laminated on both sides of a planar MEA (Membrane Electrode Assembly), is regarded as one unit, and plural units are stacked and form a fuel cell stack. The MEA is formed as a three-layer structure in which an electrolyte membrane made of, for example, an ion exchange resin or the like, is interposed between a pair of gas diffusion electrodes that constitute a cathode and an anode. In the gas diffusion electrode, a gas diffusion layer is formed outside of an electrode catalyst layer in contact with the electrolyte membrane. Furthermore, the separator is laminated so as to come into contact with the gas diffusion electrode of the MEA and a gas passage that allows a gas to flow and a coolant passage are thereby formed between the separator and the gas diffusion electrode. According to such a fuel cell, for instance, when a hydrogen gas as a fuel is caused to flow in the gas passage facing the gas diffusion electrode on the anode side and an oxidizing gas such as oxygen or air is caused to flow in the gas passage facing the gas diffusion electrode on the cathode side, an electrochemical reaction occurs, resulting in the generation of electricity.
The above separator must have a function in which electrons generated by catalytic reaction of the hydrogen gas at the anode side are supplied to an external circuit, and in addition, the electrons are supplied from the external circuit to the cathode side, and therefore, materials having conductivity are used as a material thereof. A conventional separator is made of graphite, graphite-resin composition material, etc., and the above gas passage and coolant passage are formed by cutting, molding, or the like.
In the separator, very high corrosion resistance is required since a conditions inside the fuel cell, in particular, the conditions in the above gas passage and coolant passage, etc., are humid and of low pH. The graphite type materials are superior in this respect; however, they have disadvantages in that a press-molding which is superior in productivity cannot be employed for producing the separator. Thus, currently, as a separator, a material using a stainless steel plate which can employ the press-molding and is superior in corrosion resistance, is being produced. In such separator, a stainless steel plate is rolled so as to be a thin plate, this thin plate is press-molded into a corrugated form in cross section so that grooves are formed on the upper surface and the rear surface thereof, and the above gas passages and coolant passages are thereby formed.
However, stainless steel may have defects such as pin holes in a passivation film which is formed naturally on the surface thereof in air, and corrosion originates thereat, and therefore, it cannot be said that stainless steel has sufficiently corrosion resistance, and it is necessary to further improve the corrosion resistance. In contrast, when the corrosion resistance is excessively increased, there is a problem in that contact resistance for the electrode construction increases and performance of electrical generation is reduced.